The present invention relates to froth flotation processes for recovery of mineral values from base metal sulfide ores. More particularly, it relates to new and improved sulfide mineral depressants for use in separating or beneficiating sulfide minerals by froth flotation procedures, and to a new and improved process for beneficiating sulfide minerals by froth flotation incorporating these and other depressants.
Certain theory and practice state that the success of the sulfide flotation process depends to a great degree on reagents called collectors that impart selective hydrophobicity to the mineral value which has to be separated from other minerals.
Certain other important reagents, such as the modifiers, are also largely responsible for the success of flotation separation of the sulfide and other minerals. Modifiers include but are not necessarily limited to all reagents whose principal function is neither collecting nor frothing, but usually one of modifying the surface of the mineral so that a collector either adsorbs to it or does not. Modifying agents may thus be considered as depressants, activators, pH regulators, dispersants, deactivators, etc. Often, a modifier may perform several functions simultaneously. Current theory and practice of sulfide flotation again state that the effectiveness of all classes of flotation agents depends to a large extent on the degree of alkalinity or acidity of the ore pulp. As a result, modifiers that regulate the pH are of great importance. The most commonly used pH regulators are lime, soda ash and, to a lesser extent, caustic soda. In sulfide flotation, however, lime is by far the most extensively used. In copper sulfide flotation, which dominates the sulfide flotation industry, for example, lime is used to maintain pH values over 10.5, more usually above 11.0 and often as high as 12 or 12.5. In prior art sulfide flotation processes preadjustment of the pH of the pulp slurry to 11.0 and above is necessary to depress the gangue sulfide minerals of iron, such as pyrite and pyrrhotite. The costs associated with adding lime are becoming quite high and plant operators are interested in flotation processes which require little or no lime addition, e.g., flotation processes which are effectively conducted at slightly alkaline, neutral or even at acid pH values. Neutral and acid circuit flotation processes are particularly desired because pulp slurries may be easily acidified by the addition of sulfuric acid, and sulfuric acid is obtained in many plants as a byproduct of the smelters. Therefore, flotation processes which require preadjustment of pH to neutral or acid pH values using less expensive sulfuric acid are preferable to current flotation processes, which presently require pH preadjustment to highly alkaline values of at least about 11.0 using lime which is more costly.
As has been mentioned above, lime consumption in individual plants may vary anywhere from about one pound of lime per metric ton of ore processed, up to as high as 20 pounds of lime per metric ton of ore. In certain geographical locations, such as South America, lime is a scarce commodity, and the current costs of transporting and/or importing lime has risen considerably in recent years. Still another problem with prior art high alkaline processes is that the addition of large quantities of lime to achieve sufficiently high pH causes scale formation on plant and flotation equipment, thereby necessitating frequent and costly plant shutdowns for cleaning.
It is apparent, therefore, that there is a strong desire to reduce or eliminate the need for adding lime to sulfide flotation processes to thereby provide substantial savings in reagents costs. In addition, reducing or eliminating lime in sulfide ore processes will provide other advantages by facilitating the operation and practice of unit operations other than flotation, such as fluids handling or solids handling, as well as the improved recovery of secondary minerals.
In general, xanthates and dithiophosphates are employed as sulfide collectors in the froth flotation of base metal sulfide ores. A major problem with these sulfide collectors is that at pH's below 11.0, poor rejection of pyrite or pyrrhotite is obtained. More particularly, in accordance with present sulfide flotation theory, the increased flotation of pyrite at a pH of less than 11 is attributed to the ease of oxidation of thio collectors to form corresponding dithiolates, which are believed to be responsible for pyrite flotation.
In addition to attempts at making the sulfide collectors more selective for value sulfide minerals, other approaches to the problem of improving the flotation separation of value sulfides have included the use of modifiers, more particularly depressants, to depress the non-value sulfide minerals and gangue minerals so that they do not float in the presence of collectors, thereby reducing the levels of non-value sulfide contaminants reporting to the concentrates. As has been mentioned above, a depressant is a modifier reagent which selectively prevents or inhibits adsorption of the collectors onto certain of the mineral particles surfaces present in the flotation slurry or pulp. Prior art sulfide depressants have been generally selected from highly toxic inorganic compounds such as sodium cyanide, (NaCN), sodium hydro sulfide, (NaSH), and Nokes reagent (P.sub.2 S.sub.5 and NaOH). These conventional sulfide depressants present a number of serious problems and have serious shortcomings attendant with their use. The conventional depressants are extremely toxic and are associated with a terrible stench. They cannot be used safely over a wide range of pH values, but instead must be used at high pH values, so that lime consumption problems are not solved by their use. Moreover, the conventional inorganic depressants are either nonselective or when used in sufficient quantities to provide good separation, provide economically unsatisfactory recoveries, i.e., the yield of value minerals is too low.
The problem facing flotation beneficiation methods today is to provide value mineral concentrates which contain substantially reduced levels of gangue sulfide minerals. The flotation concentrates are generally delivered to the smelting operations without any further substantial processing. Large amounts of sulfur dioxide are emitted from the smelters during the smelting of sulfide concentrates; a significant amount of SO.sub.2 is from the gangue sulfide minerals such as iron sulfides, which invariably report to the smelters as contaminants in the flotation concentrates. SO.sub.2 pollution of the atmosphere has always been a serious problem because it is a major cause for acid rain, which has a devastating effect on the ecology. Despite significant advances in smelting technology, SO.sub.2 pollution remains extremely serious.
In addition to the above-mentioned problems in sulfides processing, certain unique problems exist in the case of complex sulfides processing in terms of separation of one value mineral from others. Complex sulfide ores are an important source of many base metals and precious metals. It is quite common to find 3-5 metals in each deposit, in addition to Au, Ag and impurity elements such as Sb, As, Bi and Hg. The treatment method depends on the relative proportions of the different metals, but the more widely used routes are: (a) bulk flotation of sulfides followed by separation of value sulfides, and (b) differential flotation of sulfides. It is necessary to characterize each complex sulfide deposit quantitatively and systematically and then to select the economically optimum combination of process steps to suit the characteristics. Depressants are invariably used in all stages of flotation. Lime, sodium or zinc cyanide, zinc sulfate (often in combination with sodium cyanide), SO.sub.2, dichromate, dextrin, hypochlorite, and ferro cyanide are some of the most commonly used depressants.
The benefication criteria for treating the complex sulfide ores are maximum value metal and precious metals (if any) recovery and minimum contamination of the value sulfide concentrate by non-value sulfide minerals. In many cases, these criteria cannot be met without seriously sacrificing value metals production or recovery. Therefore, there remains an urgent need for flotation reagents that can selectively depress gangue sulfide minerals reporting to the concentrate and concurrently provide economically acceptable recoveries of value sulfide minerals.
Unexpectedly, in view of the foregoing, it has now been discovered that certain synthetic polymers which contain certain functional groups are very selective depressants for pyrite, pyrrhotite, and other gangue sulfide minerals. The use of the depressants of the present invention provides a substantial reduction in gangue sulfide minerals contamination in the sulfide minerals concentrates reporting to the smelters, thereby reducing the adverse environmental impact of SO.sub.2 emissions caused by smelting operations in the industry.